Bleaching procedure using chlorine dioxide and chlorine solutions

ABSTRACT

The advantages of the serial application of chlorine dioxide and chlorine without intermediate wash during the bleaching of wood pulp in the absence of significant amounts of dissolved organic materials are improved by the presence of small quantities of dissolved chlorine in the chlorine dioxide solution.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser.No. 293,592 filed Aug. 17, 1981, which is a continuation of applicationSer. No. 098,524 filed Nov. 29, 1979 (now U.S. Pat. No. 4,299,653 issuedNov. 10, 1981), which is a continuation-in-part of application Ser. No.030,557 filed Apr. 16, 1979 (now abandoned).

FIELD OF INVENTION

The present invention relates to an improved bleaching sequence.

BACKGROUND TO THE INVENTION

It has previously been suggested in U.S. Pat. No. 3,536,577 to effectbleaching of cellulosic fibrous material, particularly wood pulp, usingan aqueous solution of chlorine dioxide and chlorine at moderatetemperatures up to 35° C. In this process, the chlorine dioxideconstitutes about 20 to about 95% of the total available chlorine of thesolution.

It has also previously been found that the characteristics of the pulpbleached by the latter process can be improved by applying the chlorinedioxide and chlorine sequentially instead of as a mixture of the two,without an intermediate washing between the two bleaching chemicalapplications, as described in U.S. Pat. Nos. 3,433,702 and 3,501,374.

SUMMARY OF INVENTION

It has now been surprisingly found that the efficiency ofdelignification of the bleached pulp is further improved if part of thechlorine is included with the chlorine dioxide in the first step of thebleaching chemical application.

In accordance with the present invention, there is provided a multi-stepprocess, wherein a gaseous mixture of chlorine dioxide, chloride andsteam is formed using a specific procedure, an aqueous solution ofchlorine dioxide and chlorine having a specific concentration range isformed from the gaseous mixture, and a cellulosic fibrous material pulpis bleached in an aqueous suspension thereof at a specific consistencyusing the aqueous solution, in a serial application with an aqueouschlorine solution without an intermediate washing step under specificconditions.

To distinguish the bleaching chemical application procedure of thepresent invention from that previously described, the application of anaqueous solution of chlorine dioxide and chlorine, followed, without anintermediate wash, by chlorine is termed "serial bleaching" and theprevious process wherein chlorine dioxide is followed by chlorinewithout an intermediate wash is termed "sequential bleaching".

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graphical representation of the variation in pulp E₁ Kappavalues with chlorine content of an aqueous solution of chlorine dioxideand chlorine used in serial bleaching chemical application at differingdissolved organic material levels;

FIG. 2 is a graphical representation of a similar variation to FIG. 1for a different equivalent Cl₂ level in the bleaching step; and

FIG. 3 is a flow sheet illustrating one embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION

Referring to FIG. 3 which is a flow sheet of the current best mode ofeffecting the invention known to the applicants, a chlorine dioxidegenerator 10 produces a gaseous product stream 12 containing chlorinedioxide, chlorine and steam.

The chlorine dioxide generator 10 has a unilocular reaction zonecontaining an aqueous acid reaction medium which is maintained at atemperature of about 55° to about 85° C. while a subatmospheric pressureof about 80 to about 300 mm Hg is applied to the reaction zone tomaintain the same at its boiling point. An air bleed in line 14 isprovided.

Reactants are continuously fed to the reaction medium in the form ofsodium chlorate in line 16, sodium chloride in line 18 and sulphuricacid is fed by line 20. Hydrochloric acid may be used to replace all orpart of the sodium chloride, in which case the hydrochloric acid alsoreplaces part of the sulphuric acid. The feed of sulphuric acidmaintains the total acid normality of the reaction medium in the rangeof about 2 to about 4.8 normal. Once saturation of the reaction mediumis reached after start up, anhydrous neutral sodium sulphateprecipitates from the reaction medium and is removed by line 22.

The reactions which occur in the chlorine dioxide generator 10 may berepresented by the equations:

    NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 →ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+Na.sub.2 SO.sub.4                                       (1)

    NaClO.sub.3 +5NaCl+3H.sub.2 SO.sub.4 →3Cl.sub.2 +3H.sub.2 O+3Na.sub.2 SO.sub.4                                      (2)

The reaction of equation (1) predominates in the process and the extentto which that reaction predominates constitutes the efficiency of thechlorine dioxide producing process.

The production of chlorine dioxide by the latter procedure hasconsiderable benefits over prior chlorine dioxide-producing processesand these have lead to wide commercial practise of the processthroughout the world. The process is known as the ERCO R3 (trademarks)process and is the subject of U.S. Pat. No. 3,864,456, assigned to theassignee of this application.

Although the invention is described with particular reference to the R3process, the invention is applicable to and effective with otherchlorine dioxide and chlorine producing processes wherein added chlorideion is used as the reducing agent for chlorate in the aqueous acidreaction medium while the reaction medium is maintained at its boilingpoint under a subatmospheric pressure, whereby the chlorine dioxide andchlorine is admixed with steam as the diluent gas therefor.

Examples of other chlorine dioxide and chlorine producing processeswhich may be used in place of the procedure of U.S. Pat. No. 3,864,456are the ERCO R5 (trademarks) process, described in Canadian Pat. No.913,328 and U.S. Pat. No. 4,075,308, and the ERCO R6 (trademarks)process, described in U.S. Pat. No. 3,929,974, wherein the sodiumchlorate is reduced by hydrochloric acid, which provides all thechloride ions and all the hydrogen ions required for the process. Thereactions involved are summarized by the following equations:

    NaClO.sub.3 +2HCl→ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O+NaCl(3)

    6NaClO.sub.3 +6HCl→3Cl.sub.2 +3H.sub.2 O+6NaCl      (4)

wherein equations (3) and (4) correspond to equations (1) and (2) forthe R3 process.

Another chlorine dioxide and chlorine producing process which may beutilized in the present invention is the ERCO R7 (trademarks) process,described in U.S. Pat. No. 4,086,329, wherein sulphuric acid andhydrochloric acid used in the reduction reaction are mainly regeneratedinternally by reaction of chlorine with sulphur dioxide and water.

The gaseous product stream in line 12 consisting of chlorine dioxide,chlorine, steam and a small amount of air is conveyed to a condensor 24wherein the gas stream is cooled to a temperature of about 3° to about60° C. (preferably about 7° to about 60° C., to cause partialcondensation of the steam therein.

The gas stream in line 26 is fed into countercurrent contact in acontact tower 28 with a stream of chilled water in line 30 having atemperature of 0° to about 22° C., preferably about 3° to about 10° C.,to dissolve all the chlorine dioxide from the gaseous stream along withpart of the chlorine.

The aqueous solution of chlorine dioxide and chlorine leaving the tower28 in line 32 usually has a concentration of chlorine dioxide of about 8to about 9 grams per liter and a concentration of chlorine of about 1.5to about 1.8 grams per liter, the actual concentrations being dependenton the temperature of the chilled water in line 30 and the flow rate ofwater to the tower 28. At the same flow rate, the concentrations ofchlorine dioxide and chlorine increase with decreasing temperature, sothat colder rather than warmer temperatures often are preferred.

The concentration of chlorine present in the aqueous solution ofchlorine dioxide and chlorine in line 32 is always above 6.0% and up to10.0% of the total available chlorine of that solution, preferably about7 to 8%.

The term "total available chlorine" is used herein in its normal meaningin the bleaching art and refers to the total bleaching power of thesolution, chlorine dioxide having a bleaching power which is 2.63 timesthat of chlorine on a weight basis.

Solutions of such concentrations contrast markedly with chlorine dioxidesolutions obtained from other chlorine dioxide-producing processes. Inthose cases where no chloride ion is added and substantially purechlorine dioxide is formed, the aqueous solution contains a negligiblechlorine concentration. For those processes where chloride ion is addedas the reducing agent and chlorine is produced along with the chlorinedioxide, such as, in the ERCO R2 (trademarks) process, which is thesubject of U.S. Pat. No. 2,863,722, wherein air is used to dilute thegases, the chlorine dioxide solution produced by absorption in watercontains some dissolved chlorine, but the concentration is well belowthe concentration thereof in the chlorine dioxide and chlorine solutionpresent in line 32. Under typical operating conditions, the differencesin partial pressure of the gases in the processes result in an aqueoussolution of chlorine dioxide and chlorine containing 8 grams per literchlorine dioxide and 1.7 grams per liter chlorine in the case of the R3or similar process and an aqueous solution of chlorine dioxide andchlorine containing 8 grams per liter chlorine dioxide and 0.5 grams perliter chlorine in the case of the R2 process.

The reference to chlorine dioxide solutions used in the prior sequentialbleaching process containing small quantities of chlorine is to suchsolutions. Such prior "technical chlorine dioxide solutions" may containup to about 10% by weight of chlorine, corresponding to 4% on anavailable chlorine basis, although the applicants are aware that theliterature refers to values as high as 5.7% on an available chlorinebasis. The typical R2 process solution contains about 2.3% on anavailable chlorine basis.

The difference in chlorine concentrations between those provided byprior procedures and those resulting in this invention arises from thedifferences in partial pressure of chlorine dioxide and chlorine in thegaseous streams which are contacted by water in the absorption tower.

The gaseous product stream resulting from the contact tower 28 consistsof chlorine and air in line 34. The vacuum in the generator 10 isusually applied via the gaseous stream in line 34, typically afterdissolving the chlorine therefrom in a convenient aqueous medium. Avacuum pump of any desired type may be used to apply the vacuum, and thevalue of the subatmospheric pressure is controlled by the air bleed tothe generator 10 in line 14.

Usually the chlorine dioxide generator 10 is provided with sufficientcapacity that the chlorine dioxide and chlorine solution in line 32 isof sufficient volume for use in all bleaching operations requiring theuse of chlorine dioxide. For convenience of illustration, the chlorinedioxide solution in line 32 is shown as being passed to the first stage36 of a multistage bleaching and purification operation effected on awashed but otherwise untreated wood pulp in line 38, although it will beunderstood that usually only a proportion thereof passes to the firststage of the bleach plant.

The process of the present invention is applicable to any cellulosicfibrous material but is described with particular reference to thebleaching of wood pulp, preferably wood pulp produced by the Kraftprocess, i.e., wood pulp produced by digestion of wood chips in apulping liquor containing sodium hydroxide and sodium sulphide as theactive pulping chemicals.

The term "bleaching stage" as used herein refers to a pulp bleachingoperation effected between other pulp treatments, usually washings. Theterm "bleaching step" as used herein refers to a pulp bleachingtreatment effected within a bleaching stage.

In the first bleaching stage 36, the pulp suspension, of consistency ofabout 2 to about 16% by weight of pulp is bleached by a serialapplication of the bleaching solutions.

In conventional pulp mill operations, pulp is washed to free the same ascompletely as possible from spent pulping liquor so as to minimize theextent to which dissolved impurities consume bleaching chemicals. Someblack liquor "carry-over" may occur, leading to the presence ofdissolved organic material in the pulp suspension. The term "dissolvedorganic material" as used herein refers to bleaching chemical-consumingorganic material dissolved in the aqueous phase of the pulp suspensionand is measured in terms of total organic carbon (TOC).

The concentration of dissolved organic material from the source mayrange up to about 2% by weight TOC on pulp, although the concentrationusually does not exceed about 1%, by weight TOC on pulp. The presentinvention is concerned with the bleaching of pulp in a pulp suspensionwhich may contain such quantities of dissolved organic material.

The pulp suspension is subjected to a first bleaching step with thechlorine dioxide and chlorine solution fed by line 32 at a temperatureof about 35° to about 70° C., preferably about 50° to about 65° C. Thefirst bleaching step may be effected at any desired acid pH value,usually at a pulp suspension pH of about 1 to about 6.

After a period of time of from about 5 seconds to about 10 minutes,usually about 30 seconds to about 5 minutes, has lapsed from theapplication of the aqueous solution of chlorine dioxide and chlorine, anaqueous chlorine solution fed by line 40 is applied to the pulpsuspension. The chlorine solution in line 40 may be provided from anyconvenient source, for example, by absorbing the chlorine gas in line 34in water. The chlorine treatment may be effected at any desired acid pHvalue, usually at a pulp suspension pH of about 0.7 to about 3. Thebleaching procedure is allowed to proceed for about 10 to about 60minutes at the bleaching temperature of about 35° to about 70° C.

The overall amount of available chlorine applied to the pulp in thefirst and second bleaching steps, both as chlorine dioxide and chlorine,usually is about 2 to about 10% by weight of the pulp. The chlorinedioxide in the aqueous solution of chlorine dioxide and chlorineconstitutes about 20 to about 90%, preferably about 30 to about 75%, ofthe total available chlorine used in the first and second bleachingsteps.

Following completion of the bleaching stage, the pulp passes by line 42to the remainder of the multistage bleaching and caustic extractionoperations 44, wherein the pulp is subjected to EDED steps using anaqueous solution of sodium hydroxide fed by line 46 in the E-stages, andan aqueous solution of chlorine dioxide fed by line 48 in the D-stages,usually part of the chlorine dioxide and chlorine solution in line 32.Intermediate washing is usually effected between each chemicalapplication stage, using water fed by line 50. Other bleaching agents,such as, hypochlorite and peroxide may be used in the bleaching steps.The final bleached pulp is removed by line 52.

It is preferred to effect countercurrent flow of aqueous media withrespect to pulp flow through the bleaching and purification operations,as discussed above. This operation results in the presence of about 3.5to about 6.5% by weight TOC on pulp in the pulp suspension which isbleached in the first bleaching stage 36.

The bleaching and purification operations effected in 36 and 44 may becarried out by the so-called "dynamic bleaching" process, as describedin Canadian Pat. No. 783,483, in which case the washing steps usuallyare omitted.

EXAMPLES Example 1

A wood pulp of 34.2 Kappa number, a K number of 23.4 and a viscosity of28.8 cps was bleached in a series of experiments using 7.6% equivalentchlorine on pulp for 30 minutes at 3.5% consistency and 60° C., washed,caustic extracted using 3.0% NaOH on pulp for 2 hours at 12% consistencyand 70° C., and again washed. In each case, the Kappa number wasdetermined following the treatments. Chlorine dioxide and chlorine wereused in the bleaching step in the proportions of 70% of the totalavailable chlorine being provided by chlorine dioxide and 30% of thetotal available chlorine being provided by chlorine.

The series of experiments included tests conducted in the absence ofdissolved organic carbon and tests conducted in the presence ofdissolved organic carbon at various levels of % TOC on pulp. The seriesof experiments also included tests using an aqueous solution containingall the chlorine dioxide followed one minute after the applicationthereof by the serial application of all the chlorine in the bleachingstep without an intermediate wash, and tests using an initialapplication of an aqueous solution of chlorine dioxide and chlorinecontaining all the chlorine dioxide and part of the chlorine, in varyingproportions, followed one minute after the application thereof by theremainder of the chlorine without an intermediate wash.

The results of the series of experiments were plotted graphically on thebasis of E₁ Kappa number against proportion of Cl₂ in ClO₂ solution forvarying % TOC levels and the graph appears as FIG. 1. As may be seenfrom the results depicted in FIG. 1, the efficiency of delignificationof the pulp, as depicted by the Kappa number, improves when the serialapplication of the bleaching chemicals is effected using an initialaqueous solution of chlorine dioxide and part of the chlorine, whencompared with the initial application of an aqueous solution of chlorinedioxide and chlorine, over a limited range of chlorine content of theaqueous solution of chlorine dioxide and chlorine and % TOC level onpulp.

The improved efficiency is best at 0% TOC and is substantially absent at3% TOC. The improved efficiency is greatest in the range of about 5 to10% of the total available chlorine provided by Cl₂ in the aqueoussolution of chlorine dioxide and chlorine, although at 0% TOC, benefitspersist to 15%.

Example 2

The procedure of Example 1 was repeated except that in this instance, aseries of experiments was conducted at % TOC only at an equivalentchlorine level of 6.2% on pulp. The E, Kappa results attained wereplotted graphically and appear as FIG. 2. The results of FIG. 2 confirmthe beneficial results illustrated by FIG. 1.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a methodof serial application of bleaching chemicals in the absence ofsignificant amounts of dissolved organic materials wherein improveddelignification efficiency may be attained by the presence of smalldissolved quantities of chlorine in the chlorine dioxide solution.Modifications are possible within the scope of this invention.

What we claim is:
 1. A method of bleaching pulp using chlorine dioxide,which comprises:(a) continuously forming a gaseous mixture of chlorinedioxide, chlorine and steam by:(i) continuously feeding a sodiumchlorate solution to an aqueous acid reaction medium present in aunilocular reaction zone, (ii) continuously feeding sulphuric acid tosaid aqueous reaction medium in an amount to maintain the total acidnormality of the reaction medium in the range of about 2 to about 4.8normal, (iii) continuously feeding sodium chloride, hydrochloric acid ora mixture of sodium chloride and hydrochloric acid to said aqueousreaction medium, (iv) continuously maintaining said reaction medium at atemperature of about 55° to about 85° C. while maintaining said reactionzone under a subatmospheric pressure of about 80 to about 300 mm Hg tocause the formation of chlorine dioxide and chlorine and the evaporationof water from the reaction medium, (v) continuously depositing anhydrousneutral sodium sulphate from the reaction medium in said reaction mediumin said reaction zone once the reaction medium becomes saturated therebyafter start up, and (vi) continuously removing the gaseous mixture ofchlorine dioxide, chlorine and steam from the reaction zone; (b)continuously cooling said gaseous stream to a temperature of about 3° toabout 60° C. to cause condensation of at least a substantial proportionof the steam therefrom to provide a chlorine dioxide- andchlorine-containing gas stream; (c) continuously contacting the lattergas stream with water having a temperature of about 0° to about 22° C.at a flow rate sufficient to form an aqueous solution of chlorinedioxide and chlorine containing chlorine in an amount of 6 to 10% of theavailable chlorine of the solution, and a gaseous chlorine stream; and(d) bleaching a cellulosic fibrous material pulp for about 10 to about60 minutes at a temperature of about 35° to about 70° C. in an aqueoussuspension having a consistency of about 2 to about 16% by weight ofpulp and containing no more dissolved organic material than up to about2% by weight TOC on pulp at an overall equivalent chlorine concentrationof about 2 to about 10% by weight of the pulp, by:(i) subjecting saidsuspension to a first bleaching step at an acid pH value using at leastpart of said aqueous solution of chlorine dioxide and chlorine formed instep (c), and (ii) without an intermediate washing step, subjecting thesuspension to a second bleaching step at an acid pH using a chlorinesolution formed from at least part of the gaseous chlorine stream fromStep (c) and commencing about 5 seconds to about 10 minutes aftercommencement of said first bleaching step,the chlorine dioxide in saidaqueous solution of chlorine dioxide and chlorine constituting about 20to about 90% of the total available chlorine used in said first andsecond bleaching steps.
 2. The method of claim 1 wherein said gaseousstream is continuously cooled to a temperature of about 7° to about 60°C.
 3. The method of claim 1 wherein said water has a temperature ofabout 3° to about 10° C.
 4. The method of claim 1 wherein said gaseousstream is cooled to a temperature of about 7° to about 60° C. to effectsaid condensation and said water has a temperature of about 3° to about10° C.
 5. The method of claim 1, 2, 3 or 4 wherein said chlorine dioxideand chlorine solution contains chlorine in an amount of about 7 to 8% ofthe available chlorine of the solution.
 6. The method of claim 1, 2, 3or 4 wherein said aqueous solution of chlorine dioxide and chlorinecontains about 8 to about 9 grams per liter of chlorine dioxide andabout 1.5 to about 1.8 grams per liter of chlorine.
 7. The method ofclaim 1 wherein said dissolved organic material content of saidsuspension is up to about 1.0% by weight TOC on pulp.
 8. The method ofclaim 1 wherein said cellulosic fibrous material is wood.
 9. The methodof claim 1 wherein said first and second bleaching steps are effected ata temperature of about 50° to about 65° C. and the second bleaching stepis commenced about 30 seconds to about 5 minutes after commencement ofthe first bleaching step.
 10. The method of claim 1 wherein saidchlorine dioxide in said aqueous solution of chlorine dioxide andchlorine constitutes about 50 to about 75% of the total availablechlorine used in said first and second bleaching steps.
 11. The methodof claim 1 wherein said pulp suspension has a pH of about 1 to about 6in said first bleaching step and a pH of about 0.7 to about 3 in saidsecond bleaching step.
 12. The method of claim 1 wherein:(a) saidcellulosic fibrous material is wood; (b) said first and second bleachingsteps are effected at a temperature of about 50° to about 65° C.; (c)said second bleaching step is commenced about 30 seconds to about 5minutes after commencement of the first bleaching step; (d) said firstbleaching step is effected at a pulp suspension pH of about 1 to about6; (e) said second bleaching step is effected at a pulp suspension pH ofabout 0.7 to about 3; and (f) said chlorine dioxide in said aqueoussolution of chlorine dioxide and chlorine constitutes about 30 to about75% of the total available chlorine used in said first and secondbleaching steps.
 13. The method of claim 12 wherein said dissolvedorganic material content of said suspension is up to about 1.0% byweight TOC on pulp.